It would be funny if you actually did give your girlfriend a beautifully wrapped box for her birthday, and when she unwrapped it it turned out that it really was just an empty box with the words "Cat Trap" written on it with a sharpie marker. And then she playfully punches you on the arm for the joke and throws the box on the floor... and then the cat saunters over to it and climbs in and gets all comfy and cute in it. And then your girlfriend says "awwww".